With regard to an automatic transmission mounted in a vehicle such as an automobile, a starting stage such as forward first-speed has been conducted by fastening of one friction element and engagement of one one-way clutch. However, the one-way clutch is heavy and causes drag resistance in transmission stages other than the starting stage. Therefore, as disclosed in Patent Document 1, it is known that the starting stage is implemented by fastening of two friction elements without the one-way clutch. Patent Document 1 discloses that a double acting type friction, element including a clearance adjustment piston (referred to as “13 piston”) and a pressing piston (referred to as “A piston”) is used as one of the two friction elements fastened in the starting stage to improve controllability.
With regard to the double acting type friction element, the B piston is provided and moves in a transmission case. The A piston is provided in the B piston to cause relatively movement. When hydraulic pressure is supplied to a hydraulic chamber (referred to as “B chamber”) of the B piston to move the B piston, the A piston comes into contact with a friction plate so that the clearance of the friction plate decreases to zero (the hydraulic pressure of the B chamber at this time is referred to as “first hydraulic pressure”). When hydraulic pressure is supplied to a hydraulic chamber of the A piston (referred to as “A chamber”) in this state, the A piston presses the friction plate so that the double acting type friction element is set to a fastened state (the hydraulic pressure of the A chamber at this time is referred to as “second hydraulic pressure”). When the hydraulic pressure of the A chamber (i.e. the second hydraulic pressure) dissipates in this state, the A piston stops pressing the friction plate so that the double acting type friction element is set to a released state. In this way, the double acting type friction element is responsively fastened by the A piston on standby in a position where the clearance of the friction plate is zero (“referred to as “zero clearance position”). There is a decrease in a fastening shock.
With regard to a vehicle (referred to as “idling stop vehicle”) which conducts idling stop control to automatically stop an engine when predetermined stop conditions are satisfied and restart the engine when predetermined restart conditions are satisfied in an engine automatic stop state, there are the following problems when the double acting type friction element is fastened in the starting stage.
It is desirable for an electric oil pump to supply hydraulic pressure in advance to hydraulic chambers of friction elements fastened in the starting stage while the engine is stopped automatically by the idling stop control so that the vehicle may quickly start during the engine restart. For example, two friction elements fastened in the starting stage (forward first-speed stage) of a D range are a friction element (referred to as “first friction element”) including a single piston and a single hydraulic chamber, and the double acting type friction element (referred to as “second friction element”) described above. While the engine is stopped automatically in the D range, there is a supply of hydraulic pressure to the first friction element for changing the first friction element to a fastened state and a supply of the first hydraulic pressure to the B chamber of the second friction element whereas the hydraulic pressure of the A chamber dissipates. Consequently, the first friction element is set to the fastened state whereas the A piston of the second friction element is set to a standby state in the zero clearance position. The second friction element is responsively fastened to make the starting stage responsive simply by a supply of the second hydraulic pressure to the A chamber. Therefore, the vehicle may be quickly started during the engine restart.
A range may be switched from the D range to the P range while the engine is stopped automatically after the vehicle stops in the D range and the engine automatically stops. It is desirable to surely block a power transmission route and avoid generation of a driving force while the engine is stopped automatically in the P range, for a failsafe purpose. Therefore, the hydraulic pressure of the first friction element dissipates in a hydraulic pressure control state in the D range when the range is switched to the P range during the automatic engine stop in the D range. In short, both of the two friction elements fastened in the starting stage of the D range are changed to the released state.
The engine may be restarted by a subsequent P→D selecting operation during an automatic stop of the engine in the P range. The first friction element is fastened, and then the second friction element excellent in controllability is fastened when the starting stage of the D range is conducted during the engine restart. There may be high controllability of fastening control with a suppressed shock. The first friction element fastened earlier includes a return spring that urges a piston to a release side. The piston has to move to the fastening side under compression of the return spring against an urging force of the return spring in order to fasten the first friction element. This requires sufficiently high hydraulic pressure. However, hydraulic pressure generated by a mechanical oil pump is still relatively tow immediately after the engine restart, the mechanical oil pump being driven by the engine. Therefore, it takes time to move the piston against the urging force of the return spring. In other words, it takes time to fasten the first friction element. Consequently, even if the A piston of the second friction element is put on standby in the zero clearance position, the fastening of the second friction element is delayed so that the starting stage implementation becomes less responsive.
The same problem may happen, for example, when P→R selecting operation is performed during an automatic stop of the engine in the P range, when N→D selecting operation or N→R selecting operation is performed during an automatic stop of the engine in the N range, and when D→R selecting operation is performed during an automatic stop of the engine in the D range.